Passenger vehicles include a powertrain that is comprised of an engine, a multi-speed transmission, and a differential or final drive. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. The number of forward speed ratios that are available in the transmission determines the number of times the engine torque range is repeated. Early automatic transmissions had two speed ranges. This severely limited the overall speed range of the vehicle and therefore required a relatively large engine that could produce a wide speed and torque range. This resulted in the engine operating at a specific fuel consumption point during cruising, other than the most efficient point. Therefore, manually-shifted (countershaft transmissions) were the most popular.
With the advent of three- and four-speed automatic transmissions, the automatic shifting (planetary gear) transmission increased in popularity with the motoring public. These transmissions improved the operating performance and fuel economy of the vehicle. The increased number of speed ratios reduces the step size between ratios and therefore improves the shift quality of the transmission by making the ratio interchanges substantially imperceptible to the operator under normal vehicle acceleration.
It has been suggested that the number of forward speed ratios be increased to six or more. Six-speed transmissions are disclosed in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 31, 1978; and U.S. Pat. No. 6,422,969 issued to Raghavan and Usoro on Jul. 23, 2002.
Six-speed transmissions offer several advantages over four- and five-speed transmissions, including improved vehicle acceleration and improved fuel economy. While many trucks employ power transmissions having six or more forward speed ratios, passenger cars are still manufactured with three- and four-speed automatic transmissions and relatively few five or six-speed devices due to the size and complexity of these transmissions.
Seven speed transmissions are disclosed in U.S. Pat. No. 6,623,397 issued to Raghavan, Bucknor and Usoro. Seven speed transmissions provide further improvements in acceleration and fuel economy over six speed transmissions. However, like the six speed transmissions discussed above, the development of seven speed transmissions has been precluded because of complexity, size and cost.
Typically, vehicles classified as dual usage on and off road operation, incorporate a transfer case with two speeds (typically one reduction speed ratio and one direct speed ratio) and a drop gear set used to transfer drive power to the front axles. The engagement of the transfer case employs a mechanical engagement device which has limited controllability in comparison to an electronically controlled transmission with torque-transmitting mechanisms engaged and disengaged via signals provided from an electronic control unit.